Electrophotographic apparatus and method for imagewise charge generation and transfer

ABSTRACT

An electrophotographic apparatus and method for image-wise generating electrostatic charges from a unique electrophotographic recording element and for simultaneously imagewise transferring such charges to a insulative receiving sheet to form a developable latent electrostatic image thereon. The recording element comprises a layer of radiation-responsive material, such as a photoconductor, which is disposed between a conductive backing and a fine conductive grid member, the latter being separated from the radiation-responsive layer by a small air-gap. Imagewise charge generation is accomplished by imagewise exposing the radiation-responsive layer of the recording element to actinic radiation while creating a sufficient electric field between the conductive backing of the recording element and the grid member to produce an ionic or corona discharge between the irradiated portions of the radiation-responsive layer and the grid member. Simultaneous imagewise charge transfer is accomplished by spacedly arranging the insulative receiving sheet relative to the grid member of the recording element and creating a second electric field between the grid member and an electrode disposed behind the receiving sheet, whereby charges generated during exposure of the electrographic plate are immediately drawn to the insulative receiving sheet.

United States Patent 191 Weiss ELECTROPHOTOGRAPHIC APPARATUS AND METHOD FOR IMAGEWISE CHARGE GENERATION AND TRANSFER [75] Inventor: Armin K. Weiss, Rochester, N.Y.

[73] Assignee: Eastman Kodak Company,

Rochester, N.Y.

[22] Filed Mar. 3, 1972 [21] Appl. No.: 231,696

[52] US. Cl. 96/1 R, 96/l.3, 96/1.1, 96/1.5, l17/17.5, 117/21, 355/39, 250/65 ZE, 346/74 EB, 346/74 P, 346/74 TP [51] Int. Cl. G03g 13/00, (103g 13/08 [58] Field of Search 96/1, 1.1, 1.3, 1.4,

[56] References Cited UNITED STATES PATENTS 2,758,525 8/1956 Moncriess-Yeates 95/l.3 2,825,814 3/1958 Walkup 250/49.5 3,066,298 11/1962 McNaney 346/74 3,071,645 1/1963 McNaney 178/6.6

3,185,051 5/1965 Goffe 95/l.7 3,329,500 7/1967 Goffe..... 96/1.1 3,458,700 7/1969 Kohashi. 250/71 3,464,818 9/1969 Waly 96/1.3

Primary Examiner-George F. Lesmes Assistant Examiner-M. B. Wittenberg Attorney-Robert W. Hampton et al.

[ Nov. 13, 1973 [57] ABSTRACT An electrophotographic apparatus and method for image-wise generating electrostatic charges from a unique electrophotographic recording element and for simultaneously imagewise transferring such charges to a insulative receiving sheet to form a developable latent electrostatic image thereon. The recording element comprises a layer of radiation-responsive material, such as a photoconductor, which is disposed between a conductive backing and a fine conductive grid member, the latter being separated from the radiationresponsive layer by a small air-gap. lmagewise charge generation is accomplished by imagewise exposing the radiation-responsive layer of the recording element to actinic radiation while creating a sufficient electric field between the conductive backing of the recording element and the grid member to produce an ionic or corona discharge between the irradiated portions of the radiation-responsive layer and the grid member. Simultaneous imagewise charge transfer is accomplished by spacedly arranging the insulative receiving sheet relative to the grid member of the recording element and creating a second electric field between the grid member and an electrode disposed behind the receiving sheet, whereby charges generated during exposure of the electrographic plate are immediately drawn to the insulative receiving sheet.

15 Claims, 6 Drawing Figures air/2.010

m nimum 1 ms SHEET 2 BF 3 kl/l/l/l/ ZEEEEEEZE HM AH ELECTROPHOTOGRAPHIC APPARATUS AND METHOD FOR IMAGEWISE CHARGE GENERATION AND TRANSFER BACKGROUND OF THE INVENTION The present invention relates to novel electrophotographic recording elements, as well as to electrophotographic recording methods and apparatus wherein electrostatic charges are imagewise generated and transferred from an electrophotographic recording element to a receiving sheet whereon they may be rendered visible and permanentized.

Electrophotography, as practiced today, commonly includes the separate and distinct steps of uniformly charging the radiation-responsive layer of an electrophotographic recording element to uniformly sensitize such layer to radiation of some sort; imagewise exposing the charged layer to actinic radiation to selectively dissipate the uniform charge carried thereby, leaving behind a developable electrostatic image; developing such electrostatic image by applying electroscopic toner particles thereto; and transferring the developed or toned image to a receiving sheet whereon the toned image can be permanentized or fused to provide a hard As evidenced by the multitude of patents relating to the electrophotographic art and the numerous articles and texts which havebeen published in this area, considerable effort has been expended heretofore in improving or refining the electrophotographic reproduc tion process and apparatus, first disclosed by Chester F. Carlson more than three decades ago. One area wherein considerable attention has been focused involves the image-wise transfer of electrostatic charges from an electrophotographic recording element, across a small air-gap, to a dielectric or insulative receiving sheet. Techniques for accomplishing such charge transfer are referred to in the art as TESI processes, an acronym derived from Transfer of ElectroStatic Images. A particularly intriguing TESI process, from a scientific as well as a commercial standpoint, is one wherein the electrostatic image is both generated and transferred while the dielectric receiving sheet is arranged in a face-to-face relationship with a conventional electrophotographic plate, separated therefrom by a minute air-gap. In such a process, the normally separate steps of charging, imagewise exposing, and transferring the image (in this case a charge image, rather than a toner image) to the ultimate receiving sheet are accomplished substantially simultaneously. Such a process is disclosed in U.S. Pat. No. 2,825,814 issued to L. Walkup. Imagewise charge generation and transfer are accomplished by simultaneously applying an electrical potential between the conductive backing of the electrophotographic recording element and an electrode disposed on the rear surface of the receiving sheet, and imagewise exposing the photoconductive layer of the recording element, either through a transparent conductive backing or through the receiving sheet, to actinic radiation. During exposure, an electrostatic image is formed on the dielectric receiving sheet by an inductive transfer of charge from the plate across the small air-gap separating the dielectric and photoconductive surfaces.

While capable of providing high quality copies under controlled conditions in the laboratory, TESI processes of the type referred to above have not been found commercially practical. A major difficulty, of course, stems from the requirement of repetitively establishing a uniform minute air-gap, typically ten microns wide, between the electrophotographic recording element and the receiving sheet each time a copy is made. When the air-gap is too great, charge transfer cannot occur due to the reduction in electric field; on the other hand, when the gap is too small for the applied potential, areing occurs. While considerable work has been directed at this problem, no economically feasible solution has been found to date.

Added to the air-gap problem of conventional TESI processes is a self-quenching effect which limits the amount of charge generation and transfer. In conventional systems, the maximum potential which can be applied between the recording element and the receiving sheet is typically of the order of 1,000 volts. Higher voltages will produce arcing or corona discharge across the air-gap even when the system is in the dark, or unexposed. Assuming that, during exposure, a field of say 300 volts must be applied to produce the necessary charge generation or corona discharge between the spaced members, this means that the maximum charge that can be transferred to the receiving sheet before the field in the air-gap drops below the level required for charge generation is of the order of 350 volts (i.e., onehalf of the difference between the applied field and the corona threshold voltage). Once a charge proportional to 350 volts has transferred, the field in the air-gap is insufficient to produce charges and quenching occurs.

SUMMARY OF THE INVENTION In accordance with the present invention there is provided a novel electrophotographic recording element and a method and apparatus for imagewise generating electrostatic charges and transferring such charges to a receiving surface. The recording element comprises a conductive support having a radiation-responsive layer (e.g., a photoconductive, heat deformable thermoplastic resin, or x-ray sensitive layer) disposed thereon, a fine, electrically conductive grid or mesh and means defining a small air-gap between the grid and the radiation-responsive layer. Imagewise charge generation is accomplished by establishing an electrical field across the air-gap seaprating the grid from the radiation-responsive layer, and simultaneously imagewise exposing the radiation-responsive layer to actinic radiation. The field in such air gap is adjusted so as to be slightly less than the threshold level required for corona discharge or ionic movement across the air-gap when the recording element is in the dark (i.e., unexposed). During imagewise exposure, due to the selec tive reduction in the resistivity of the radiationresponsive layer or the selective shrinkage in distance separating the spaced members of the recording element, imagewise charge generation occurs between the irradiated areas of the radiation-responsive layerand the grid. Charge transfer is accomplished by spacedly arranging a dielectric receiving sheet relative to the grid and establishing a second electric field across the air-gap separating the grid from the receiving sheet. The second field is selected to be of a polarity and strength such as to attract the charges generated during imagewise exposure of the recording element, through the grid, to the receiving sheet surface. In this manner, a developable charge image is formed on the receiving sheet. According to a preferred embodiment, an AC field is established in the air gap separating the grid from the radiation-responsive layer; thus, both positive and negative ions are generated during exposure. By simply selecting the proper polarity for the field between the grid and receiving sheet, both positive and negative electrostatic images of the original are attainable.

As will be appreciated from the ensuing detailed description of preferred embodiments, the present invention offers several major advantages over known TESI techniques. For instance, unlike conventional techniques wherein it is suggested that the receiving sheet be brought into actual physical contact with the recording element (it being assumed that there will always be an air-gap of approximately 1 micron between the surfaces at most points), the receiving sheet, in accordance with the present invention, need never contact the radiation-responsive layer of the recording element. In fact, it may be maintained as far away from the grid member of the recording element (the uppermost surface) as one-half inch at all times. Thus, no abrading or contamination of the recording element is encountered and the useful lifetime of the plate is maximized. Also, the air-gap between the corona generating grid and recording element, once established, can be permanent; i.e., it need not change each time a copy is made. It should be noted that the air-gap between the grid and receiving sheet is not critical since its sole purpose is simply to attract ions or charges which are already generated (by the grid), not to produce such ions originally. Moreover, unlike convention TESI processes, the instant invention provides a means by which contrast can be added to the electrostatic image. It will be apparent from the ensuing description that the variation in density of the original document can be greatly exaggerated simply by employing a strong field between the grid and the receiving sheet while using relatively high exposures of the electrophotographic recording element.

Other advantages of the present invention will be apparent to those skilled in the electrophotography art from the ensuing detailed description of preferred embodiments, reference being made to the accompanying groups.

BRIEF DESCRIPTION OF THE DRAWINGS Reference is now made to the accompanying drawings wherein like reference numerals and characters designate like parts and wherein:

FIG. 1 is a diagrammatic cross-sectional view of an imagewise charge generating and transferring apparatus in accordance with a preferred embodiment of the invention, also illustrating a novel electrophotographic recording element of the invention.

FIGS. 2 and 3 are similar views illustrating alternate means for providing a minute airspace between the radiation-responsive layer of the electrophotographic recording element and the grid member;

FIG. 4 is a diagrammatic cross-sectional view of an electrophotographic copier embodying the invention;

FIGS. 5 and 6 are diagrammatic cross-sectional views illustrating other preferred embodiments of the invention wherein the radiation-responsive layer of the electrophotographic recording element comprises heatdeformable and x-ray sensitive layers, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A novel electrophotographic recording element E, as well as apparatus for imagewise generating and transferring charges therefrom, is diagrammatically illustrated in FIG. 1. As shown, recording element E comprises a transparent support 10, such as a glass or quartz plate, or a flexible web of polyethylene terephthalate having a transparent conductive coating 11 disposed on at least one surface thereof. Coating 111 may comprise a thin layer of tin oxide, cuprous iodide, aluminum, silver or any other conductive material which can be coated sufficiently thin as to be substantially transparent to radiant energy. Disposed on coating 111 and in electrical contact therewith is a radiationresponsive layer 12 which may comprise, for instance, a photoconductive compound of cadmium sulfide, zinc oxide, lead oxide, selenium or any other photoconductive compound employed in electrography. Preferably, however, the radiation-responsive layer should have a bulk resistivity of less than approximately 10 ohm-cm. since compounds having a higher resistivity tend to prevent the current flow required in the operation of the novel process disclosed herein. The most useful photoconductive material has been found to be cadmium sulfide having a pigment-to-binder ratio of about 10:1.

Closely spaced to the upper surface of radiationresponsive layer 12 is an electrically conductive grid 14 which typically comprises two arrays of uniformly spaced fine wires 16, such arrays extending perpendicular to one another to form a screen-like electrode. Dielectric spacers 18 of polystyrene or the like serve to permanently establish a uniform air-gap 20 between grid 14 and layer 12. The average width of air-gap 20 may be anywhere between approximately 1 micron (i.e., virtual contact) to 1,000 microns. Preferably, however, air-gap 20 is between 10 and microns. Grid 14 acts as the charge-generating member of the apparatus and can comprise from 30 to 400 wires per inch in each direction. Preferably, the wires make up no more than 10-50 percent of the total grid area. For best results, the grid should comprise at least 200 wires per linear inch in each direction.

Spaced froni theifppefiost sEffa o f t he radiationresponsive layer 12, and extending substantially parallel thereto, is a dielectric receiving sheet 22, behind which an electrically conductive backing electrode 24 is disposed. Dielectric receiving sheet 22 may comprise, for instance, a sheet of Mylar, a polyethylenecoated paper, or the like. The spacing between the grid and receiving sheet should be between 0.08 to 0.50 inch, a spacing of 0.125 inch being preferred.

Preferably, the conductive coating 11 of recording element E is connected to grid 14 through an AC source of potential 26; however, for most applications, as explained subsequently herein, a DC source has equal utility. When unexposed to actinic radiation, the field produced by AC source 26 across air-gap 20 is slightly below the threshold value required for corona or ionic emission. Grid 14 is connected through DC source 28 to conductive electrode 24, disposed on the rear side of dielectric receiving sheet 22. In this manner, an electric field is established across the relatively large air-gap 30 separating dielectric receiving sheet 22 from grip 14.

In operation, actinic radiation from projector P is imagewise distributed on the radiationqesponsive layer 12. While exposure of layer 12 is preferably effected through the transparent support and conductive coating 11, exposure of layer 12 can be accomplished through the receiving sheet when the latter comprises a transparent dielectric material (e.g., a sheet of polyethylene terephthalate) and conductive backing 24 is also transparent. However, exposure through the receiving sheet results in a reduction in resolution of the transferred charge image due to the presence of grid 14 in the exposure path.

During imagewise exposure of layer 12, the conductivity thereof is selectively increased, thereby selectively reducing the threshold potential required for corona emission to occur across the air-gap separating the exposed areas of layer 12 from grid 14. Due to the AC field provided by AC source 26, both positive and negative ions, 31 and 32, respectively, are imagewise generated in the air-gap. Substantially simultaneously with the imagewise generation of charges between layer 12 and grid 14, a fraction of those charges having a polarity opposite that of electrode 24 is drawn upward, through grid 14, and deposited on the surface of the dielectric receiving sheet 22. The result is a latent electrostatic image on the dielectric receiving sheet which may be developed by any one of a number of conventional electrographic development techniques, and subsequently permanentized.

From the foregoing, the advantages of the present invention are readily apparent. For instance, unlike conventional TESI techniques, the strength of the electrostatic image transferred to the receiving sheet is not limited by the maximum potential which could be applied between the electrographic recording element and the dielectric recieving sheet (i.e., the grid in the case of the present invention) in the absence of exposure to actinic radiation. In accordance with the present invention, the charge transferred to the dielectric receiving sheet will continue to increase so long as the recording element is exposed, the upper limit being limited only by the potential provided by DC source 28, and the dielectric constant of the receiving sheet.

Another major advantage of the apparatus and process of the present invention is that once the grid 14 is positioned relative to the electrographic plate, thereby establishing the necessary ionization air-gap between the members, subsequent adjustment of these members relative to one another is unnecessary. The only adjustment which is necessary in making one copy after another is the positioning of the receiving sheet relative to the grid surface and, since the spacing between the grid surface and the receiving sheet is not critical, so long as the receiving sheet is sufficiently spaced as to prevent arcing and not so greatly spaced as to allow a divergence and loss of resolution in the charge pattern to the electrographic plate, the structure is heated to a temperature such as to slightly soften the dielectric particles 32. Upon cooling, the position of the grid is fixed. This spacing technique is particularly useful when grid 14 is flimsy or flexible in nature, a property which would otherwise make uniform spacing from layer 12 difficult. Particles 32 are, of course, sufficiently small as to prevent any substantial loss of resolution in charge generation.

Still another method for positioning grid 14 relative to layer 12 is illustrated in FIG. 3 wherein exposure of layer 12 is shown as occurring through a transparent receiving sheet 22 having a transparent conductive backing 24'. As shown, the grid is disposed atop a thin, transparent dielectric coating 33 which overcoats the radiation-responsive layer 12. Coating 33 may comprise, for instance, a 0.5 mil layer of polyethylene terephthalate, cellulose acetate, or the like. To provide the necessary discharge path (i.e., air-gap) between the grid and radiation-responsive layer, dielectric coating 33 is rendered uniformly porous by exposing the entire recording element to actinic radiation, such that arcing occurs through the dielectric layer 33 at many points along the grid wires, thereby producing a multitude of minute pores which extend perpendicularly toward the radiation-responsive layer. In addition to providing the necessary separation between grid 14 and layer 12, coating 33 may be doped with a dye which will absorb unwanted radiation from the corona emission. When exposing through the receiving sheet, support 10 may, of course, comprise a sheet of metallic material, such as aluminum or copper. When exposing through the recording element, support 10, as well as its conductive coating, must be transparent; however, dielectric coating 33 need not be transparent.

The invention is further illustrated by the following example:

EXAMPLE Ten grams of CdS-Powder, Type F-2l l l, (a photoconductive cadmium sulfide powder manufactured by Radio Corporation of America), was mixed thoroughly with 3 grams of copolymer styrenebutadiene I5) (30 percent solids in toluene). The mix was twice coated on a Mylar (polyethylene terephthalate) sup port, 1 mil thick, with doctor blade; first coating 3 mils wet, second coating 5 mils wet. After 48 hours of drying at room temperature, the total thickness of this coating was approximately 4.5 mils.

To form a conductive electrode on the CdS layer, aluminum was evaporated by standard procedures to a thickness of about 1 micrometer. This conductive aluminum layer was then placed on a conductive vacuum platen. An electroplated metal grid of [50 wires per inch in each direction and 60 percent open area (manufactured by Buckby-Mears) was laid flat on top of the Mylar side of the sandwich, with the edges of the grid held in position by insulating tape. Electrical connections were made to both the grid and the vacuum platen. An AC potential of 1.5kv peak-to-peak, 60I-Iz, was applied between the grip and the vacuum platen.

A transparent receiving sheet was arranged oneeighth inch from the grid. The receiving sheet comprised an 0.25 mil thick transparent sheet of Mylar with a conductive, transparent electrode of CuI deposited thereon. A DC potential of 2,000 volts was applied between the grid and the conductive electrode of the receiving sheet, the grid being connected to the positive terminal of the DC supply. While both the AC and DC potentials were applied and the systems kept in the dark, an image was projected onto the cadmium sulfide layer through the transparent receiving sheet and grid. The projected image was from a standard 35mm microfilm (negative) slide; the images were magnified in the projection step to typewriter size. The exposure was made by a two foot-candle source for one-half second. Following exposure, the AC- and DC-potentials were disconnected from the device and the Mylar receiving sheet was removed. The electrostatic latent image on the receiving member was made visible by dipping the entire receiving sheet in a liquid xerographic toner of negative polarity for approximately two seconds. In this manner, a negative copy of the projected original was produced. Such copy exhibited a clear, transparent background and good density in the image areas.

From the foregoing example, it is clear that a positive copy of the original could have been produced by simply connecting the positive terminal of the DC source to the conductive electrode on the receiving sheet, rather than to the grid. In this manner, the negative charges or ions produced by the AC source would have been transferred to the receiving sheet and, by applying a negative toner, a positive copy could have been provided.

FIG. 4 schematically illustrates a cross-section of a commercial-type electrographic copier incorporating the spacing technique illustrated in FIG. 3. As shown, a transparent cylindrical support 10' having a transparent conductive coating 1 1 is suitably journaled for rotation (by means not shown) in the direction indicated by the arrow. Disposed on the outer surface of conductive coating 11' is a radiation-responsive layer 12 which, as mentioned above, may comprise a photoconductive compound. Atop layer 12' is a dielectric layer 33' which serves to provide a uniform spacing between the radiation-responsive layer and a conductive grid 14' which is fixedly arranged on the outermost surface of dielectric layer 33'. Layer 33' may be made porous by the technique described hereinabove so as to provide a discharge path between the grid and the outer surface of layer 12'. Concentrically spaced from grid 14' is a conductive electrode 24'. Conductive electrode 24' is preferably in the form of a vacuum plate having holes therein which communicate with a vacuum pump 40 via conduit 42.

In operation, a dielectric receiving sheet 22', which can be a resin-coated paper, is advanced from a supply spool 44 to a take-up spool 46 along a predetermined path which is partially defined by the surface S of electrode 24'. Dielectric recording sheet 22 is continuously maintained in contact with conductive electrode 24' by the vacuum applied to the electrode by vacuum pump 40. Grid 14' and conductive coating 11' are connected to an AC source'of potential (not shown) to establish an alternating field therebetween. A DC potential (not shown) is applied between grid 14' and conductive electrode 24. Prior to imagewise exposure of radiation-responsive layer 12', the field established across layer 33 by the AC source is adjusted to a value slightly less than the threshold required for corona discharge or charge generation. imagewise exposure of layer 12' is accomplished through transparent support 10' and coating 11' by a conventional projection system P. Upon exposure of layer 12', the impedance thereof is selectively reduced in the exposed area causing imagewise charge generation in the vicinity of grid 14 on the side facing dielectric layer 33. Almost simultaneously with such charge generation, a fraction of the charges is imagewise drawn toward the receiving sheet 22' owing to the DC electric field between the grid 14' and electrode 24. The result is a latent electrostatic image on dielectric receiving sheet 22' which may be developed in a conventional manner at development station 55 and permanentized at fusing station 60. When conductive electrode 24 and the dielectric receiving sheet are transparent, imagewise exposure can, of course, be accomplished through the receiving sheet. However, as mentioned above, a reduction in resolution will be encountered due to the presence of grid 14 in the exposure path.

In FIG. 5, another embodiment of the invention is presented wherein the radiation-responsive layer 12 of electrographic recording element E comprises a heatdeformable material, such as a wax or thermoplastic resin having a low melting point. Upon being imagewise exposed to thermal radiation 62, the heat-struck areas expand toward grid 14, thereby selectively reducing the spacing between the grid and layer 12. By appropriately adjusting the value of AC source 26, imagewise charge generation will occur between the grid and the irradiated areas of the heat-deformable layer. Due to field produced by a DC source 28, charges are imagewise attracted to the dielectric receiving sheet 22.

In FIG. 6, still another embodiment of the invention is disclosed wherein the radiation-responsive layer 12 of electrophotographic recording element E comprises a photoconductive layer 65 having a phosphorous overcoat 66. Upon imagewise exposing the phosphorous coating to x-rays, fluorescence is stimulated therein which imagewise increases the conductivity in the photoconductive layer. Upon establishing an appropriate field between the grid 14 and the conductive support 10 (which may be a metallic sheet in the case of x-ray exposure), and a second field between the grid and backing electrode 24, an electrostatic image is substantially simultaneously formed on the receiving sheet shown in FIG. 6, the spacing between grid 14 and the upper surface of the phosphorous overcoat 66 is maintained by a thin dielectric layer, such as a 0.5 mil layer of Teflon which has been rendered porous by the technique described above.

From the foregoing it can be readily appreciated that a much improved method and apparatus have been provided for imagewise generating and transferring electrostatic charges from an electrographic recording element of novel structure to a dielectric receiving sheet. It should also be appreciated that, in describing the subject invention, certain phrases and terms have been used in a rather general sense, rather than in a strictly technical sense, to facilitate an understanding of the invention. For instance, the phrase air-gap should be interpreted herein as referring to a space or region containing any ionizable gas, not necessarily air. Similarly, the phrase radiation-responsive" as used herein to describe the imagewise exposed layer 12 of the electrographic recording element includes all materials, photoconductive, heat deformable, x-raysensitive, etc., which, when exposed to activating radia tion undergo a physical and/or electrical change which can result in a selective corona emission when employed in an environment as herein described. Similarly, the term 7 grid should be interpreted broadly since a grid could, in addition to comprising a suitable array of fine wires, comprise point electrodes, perforated materials, etc..

This invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

1 claim:

1. A method for forming an electrostatic image on a dielectric receiving surface, said method comprising the steps of:

a. spatially positioning said receiving surface relative to an electrographic recording element, said recording element comprising a radiation-responsive layer, and an electrically conductive grid member spaced between 1.0 and 1,000 microns from said layer, said receiving surface being positioned relative to the recording element so as to face the grid member thereof;

b. establishing a first electrical field across the space between said layer and said grid member while simultaneously imagewise exposing said radiationresponsive layer to activating radiation to imagewise vary the threshold potential required for corona emission between said radiation-responsive layer and said grid member, and to produce imagewise charge generation between said layer and said grid member; and

c. establishing a second electrical field between said receiving surface and said grid member to imagewise attract charges generated during imagewise exposure of said radiation-responsive layer to said receiving surface.

2. The invention according to claim 1 wherein said first electrical field is an AC field, whereby both positive and negative charges are imagewise generated during imagewise exposure of said radiation-responsive layer.

3. The invention according to claim 1 wherein said radiation-responsive layer comprises a photoconductive material.

4. The invention according to claim 1 wherein said radiation-responsive layer comprises a heatdeformable material.

5. The invention according to claim 1 wherein said radiation-responsive layer comprises contiguous first and second sublayers, said first sublayer comprising a photoconductive material, and said second sublayer comprises a phosphor-containing material which, when imagewise exposed to x-radiation, fluoresces and thereby selectively increases the conductivity of said first sublayer.

6. A process for imagewise generating charges in an electrophotographic recording element and for substantially simultaneously imagewise transferring such charges to a dielectric receiving sheet disposed on a conductive support, such recording element comprising a radiation-responsive layer having an electrically conductive backing, an electrically conductive grid member, and means defining a substantially uniform air-gap having an average width of between approximately 1.0 and 1,000 microns between said layer and grid member, said process comprising the steps of a. spacedly arranging said receiving sheet relative to said grid member;

b. electrically biasing said grid member relative to said conductive backing while simultaneously imagewise exposing said radiation-responsive layer to actinic radiation to imagewise vary the threshold potential required for corona emission between said radiation-responsive layer and said grid member, and to produce imagewise charge generation between said layer and said grid member; and

c. electrically biasing said conductive support relative to said grid member to imagewise attract charges generated during imagewise exposure of said radiation-responsive layer to said receiving sheet.

7. The invention according to claim 6 wherein electrical biasing of said grid member relative to said conductive backing is achieved by applying an AC source of potential between said grid member and said conductive backing, whereby both positive and negative charges are generated during image-wise exposure of said radiation-responsive layer.

8. The invention according to claim 6 wherein said receiving sheet is spaced from said grid member an average distance of from 1.0 to 10.0 millimeters.

9. The invention according to claim 6 wherein said radiation-responsive layer comprises a photoconductive compound.

10. The invention according to claim 6 wherein said radiation-responsive layer comprises a heatdeformable material.

11. The invention according to claim 6 wherein said radiation-responsive layer comprises contiguous first and second sublayers, said first sublayer comprising a photo-conductive material, and said second sublayer comprises a phosphor-containing material.

12. An electrophotographic process comprising the steps of a. spacedly arranging a dielectric receiving sheet relative to an electrophotographic recording element, such element comprising a photoconductive insulating layer having a conductive backing disposed on one surface thereof and an electrically conductive grid member between 1.0 and 1,000 microns from and extending substantially parallel to the other surface of said layer, said receiving sheet being arranged so as to face said grid member and be spaced therefrom an average distance of from 1.0 to 10.0 millimeters;

b. applying an electrical potential between said grid member and said conductive backing while simultaneously imagewise exposing said photoconductive layer to actinic radiation to selectively increase the conductivity of said layer and produce imagewise charge generation between said layer and said grid member; and

c. establishing an electric field between said grid member and said receiving sheet to imagewise attract charges generated during exposure of said photoconductive layer to said receiving sheet.

13. The invention according to claim 12, further comprising the step of selectively depositing toner particles on said receiving sheet to render the charge thereon visible.

14. The invention according to claim 13, further comprising the step of fusing the toner particles to said receiving sheet.

being of an intensity less than the threshold level required for ionic movement between said grid and said layer when said layer is unexposed to activating radiation;

simultaneously exposing said layer to a pattern of activating radiation to produce selective ionic movement between said grid and said layer; and

simultaneously applying a second electric field in the space between said surface and said grid to imagewise attract ions to said surface. 

2. The invention according to claim 1 wherein said first electrical field is an AC field, whereby both positive and negative charges are imagewise generated during imagewise exposure of said radiation-responsive layer.
 3. The invention according to claim 1 wherein said radiation-responsive layer comprises a photoconductive material.
 4. The invention according to claim 1 wherein sAid radiation-responsive layer comprises a heat-deformable material.
 5. The invention according to claim 1 wherein said radiation-responsive layer comprises contiguous first and second sublayers, said first sublayer comprising a photoconductive material, and said second sublayer comprises a phosphor-containing material which, when imagewise exposed to x-radiation, fluoresces and thereby selectively increases the conductivity of said first sublayer.
 6. A process for imagewise generating charges in an electrophotographic recording element and for substantially simultaneously imagewise transferring such charges to a dielectric receiving sheet disposed on a conductive support, such recording element comprising a radiation-responsive layer having an electrically conductive backing, an electrically conductive grid member, and means defining a substantially uniform air-gap having an average width of between approximately 1.0 and 1,000 microns between said layer and grid member, said process comprising the steps of a. spacedly arranging said receiving sheet relative to said grid member; b. electrically biasing said grid member relative to said conductive backing while simultaneously imagewise exposing said radiation-responsive layer to actinic radiation to imagewise vary the threshold potential required for corona emission between said radiation-responsive layer and said grid member, and to produce imagewise charge generation between said layer and said grid member; and c. electrically biasing said conductive support relative to said grid member to imagewise attract charges generated during imagewise exposure of said radiation-responsive layer to said receiving sheet.
 7. The invention according to claim 6 wherein electrical biasing of said grid member relative to said conductive backing is achieved by applying an AC source of potential between said grid member and said conductive backing, whereby both positive and negative charges are generated during image-wise exposure of said radiation-responsive layer.
 8. The invention according to claim 6 wherein said receiving sheet is spaced from said grid member an average distance of from 1.0 to 10.0 millimeters.
 9. The invention according to claim 6 wherein said radiation-responsive layer comprises a photoconductive compound.
 10. The invention according to claim 6 wherein said radiation-responsive layer comprises a heat-deformable material.
 11. The invention according to claim 6 wherein said radiation-responsive layer comprises contiguous first and second sublayers, said first sublayer comprising a photo-conductive material, and said second sublayer comprises a phosphor-containing material.
 12. An electrophotographic process comprising the steps of a. spacedly arranging a dielectric receiving sheet relative to an electrophotographic recording element, such element comprising a photoconductive insulating layer having a conductive backing disposed on one surface thereof and an electrically conductive grid member between 1.0 and 1,000 microns from and extending substantially parallel to the other surface of said layer, said receiving sheet being arranged so as to face said grid member and be spaced therefrom an average distance of from 1.0 to 10.0 millimeters; b. applying an electrical potential between said grid member and said conductive backing while simultaneously imagewise exposing said photoconductive layer to actinic radiation to selectively increase the conductivity of said layer and produce imagewise charge generation between said layer and said grid member; and c. establishing an electric field between said grid member and said receiving sheet to imagewise attract charges generated during exposure of said photoconductive layer to said receiving sheet.
 13. The invention according to claim 12, further comprising the step of selectively depositing toner particles on said receiving sheet to render the charge thereon visible.
 14. The invention according to claim 13, further comprising the step of fusing the toner particles to said receiving sheet.
 15. A method for forming an electrostatic charge pattern on an electrically insulative surface, said method comprising the steps of: positioning an insulative surface apart from and parallel to a photoconductive insulating layer; interposing a conductive grid between said surface and said layer, so as to be spaced from and extend substantially parallel to both said insulative surface and said layer, the spacing between said grid and said layer being between 1.0 and 1,000 microns; establishing a first electric field in the space between said grid and said layer, such first electric field being of an intensity less than the threshold level required for ionic movement between said grid and said layer when said layer is unexposed to activating radiation; simultaneously exposing said layer to a pattern of activating radiation to produce selective ionic movement between said grid and said layer; and simultaneously applying a second electric field in the space between said surface and said grid to imagewise attract ions to said surface. 